1. Field of the Invention
The present invention relates to an image data processing apparatus for reading an original image, and particularly, to an optimization binarizing circuit which generates an optimal binarizing threshold value when an original image is converted into a binarized image.
2. Description of the Related Art
In a conventional data processing apparatus, e.g., a document image processing apparatus which processes code data and image data, image data which is read from an original by a scanner or the like and converted into multivalued data is converted into binarized data by means of a binarizing circuit using a fixed threshold value. A conventional binarizing threshold value is a constant value during a reading operation, that value being independent from conditions of an image formed on an original. The reading density is changed by simply increasing or decreasing the fixed threshold value. Specifically, the reading density is raised (or darkened) as the binarizing threshold value is increased, while the reading density is lowered (or lightened) as the binarizing threshold value is decreased.
In a binarizing circuit using a conventional fixed threshold value, when binarizing is carried out with a high binarizing threshold value, original data of a low contrast can be reproduced while the amount of background noise being concurrently read increases. Meanwhile, when binarizing is carried out with use of a low binarizing threshold value, original data of a low contrast is blurred while the amount of background noise being concurrently read in decreases. This is a problem with respect to a conventional circuit. Further, in a conventional binarizing circuit, while original data is read in, the binarizing threshold value is manually adjusted to an optimal value to reproduce an image portion of a low contrast and to simultaneously reduce backing noise. Specifically, the image density is adjusted by an operator based on the results of obtained by reading an original, so that the binarizing threshold value in the apparatus is changed and an image of an optimal density is obtained. This leads to another problem that adjustment of the binarizing threshold value requires a manual operation.